1. Field of the Invention
This invention related generally to regeneration of volatile liquids, and more particularly, to an improved column for the separation of a volatile liquid from a mixture with another liquid of lower volatility.
2. Description of Related Art
In systems where a vapor, such as natural gas, nitrogen, steam or some other gas, is used to strip a lower boiling component (solvent) from a mixture with a higher boiling component (e.g., methanol from methanol-water mixture using natural gas), a column is used in which the stripper column diameter is usually dictated by the amount of stripping gas or vapor required to effect the separation. The amount of gas in turn is dependent on its temperature and the carrying capacity for the solvent and the concentration of the solvent in the mixture being fed to the tower.
In presently known systems in which a column is used to strip (regenerate) methanol from a methanol-water mixture, a methanol rich water stream flows down the column, which is packed with a high efficiency structured packing, (such as Sulzer BX), and a warm feed gas stream is fed from the bottom of the column to flow up the column countercurrently. During the passage through the column, the methanol from the rich methanol-water stream is vaporized into the upflowing warm gas stream. The gas stream is usually water saturated and does not pick up water. However, as it is undersaturated or free of methanol, it drives off the methanol into the vapor phase. Typically, between 50 to 60 percent of the total feed gas stream passes through the entire column, for effective regeneration of the rich methanol water stream. The remainder of the feed gas is split and bypasses the column. The amounts to be fed to the column and split are determined or set by the process operating conditions. The diameter of the column is also determined or set by the operating pressure, temperature and flow rate of the gas and liquid streams flowing through it. Usually, the gas stream is the controlling determinate of the column diameter. The depth of the packing is determined or set by the purity of a bottom water specification, with respect to methanol content, and the split of the gas flowing through the column and that which bypasses the column.
German patent number 1113684 describes a process for desorption of a gas-laden wash-liquid wherein foam in systems having a boiling liquid fed to a column for desorption is suppressed. The process applies only to systems where in the first step, the gas and liquid are flowing co-currently. Also, the patent does not show any means of reducing the column diameter, nor does it relate in any way to the present invention.
U.S. Pat. No. 4,775,395 to Rojey et al. discloses a process and column which strips methanol from a methanol-water mixture, which requires between 50 to 60 percent of the total feed gas stream to flow through the column to effectively regenerate the rich methanol-water stream. The entire 50 to 60 percent of the flow is introduced into the bottom of the column and withdrawn at the top of the column. Hence, the diameter of the column is determined by the 50 to 60 percent of the flow going through the column.
U.S. Pat. No. 3,039,845 to H. Steinrotter et al. disclosed a process of stripping (regenerating) absorbent solutions. This patent covers a splitting arrangement for air so that concentrations of solvent are maintained in the collector and the excess air bypasses directly to the heating section. However, this patent fails to disclose a split column having either a narrower diameter, or shorter split beds for regenerating a volatile liquid from a mixture with a less volatile liquid.
In the present invention, the flow through the column at any time is reduced by approximately 50 percent of that taught by the prior art. Between 25 to 30 percent of the total feed gas stream is introduced at the bottom of the column in the U.S Pat. No. 4,775,395 patent. However, in the present invention although the total bed height may remain the same, the bed is preferably split into two or more sections of equal or unequal height. At the elevation where the beds are split, a dividing means, such as a chimney tray is installed to collect the liquid from the upper bed and redistribute this liquid to the lower bed. Also, a gas outlet is provided between the top of the lower bed and the bottom of the dividing means so that all, or a portion of the gas feed introduced at the bottom is withdrawn. A second gas inlet is provided between the top of the dividing means and the bottom of the top bed to allow introduction of another 20 to 30 percent of the total gas stream. This new gas feed, together with any remaining portion of the gas stream from the lower bed, flows up through the column and leaves from the top of the column.
Therefore, there exists a need in the art for an improved column for regenerating a volatile liquid from a mixture with a less volatile liquid, that may be made at a significantly lower cost, and which produces the same, if not improved results.